Poikiloblastic amphibole schists from an ophiolite series in the vicinity of the Tertiary Bergell granite contain the assemblage albite (An 0)–anorthite (An 95), actinolite–pargasite, together with clinozoisite. TEM illustrates how large andesine crystals with a periodic APB (anti-phase boundaries) structure of e plagioclase change over a few hundred angstroms to pure albite and pure P1 anorthite. Anorthite formation from andesine is accompanied by changes in the APB microstructure. Adjacent APB’s combine, form loops, and finally annihilate to reduce strain energy along the boundaries. The continuous transition from e to b plagioclase and the chemical phase separation are cooperative processes. APB’s provide favorable surfaces for large-scale diffusion. The TEM investigation documents that anorthite is definitely not a relict from an earlier high-grade metamorphic episode and thus appears to be stable at low metamorphic grade together with albite, which puts new constraints on the plagioclase phase diagram. Albite–anorthite assemblages are restricted to amphibole schists with pargasitic hornblende coexisting with actinolite. A striking correlation of (Na + K) and AlIV suggests that the amphibole assemblage also is the result of decomposition.

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